The use of fluidized beds has long been known as a way of improving the performance of a combustor. Where fuel is combusted in a mass of particulates forming the bed material, it is not uncommon to introduce limestone or dolomite as bed materials to combine or react with the sulfur dioxide released by the combustion reaction. The reaction in the combustion chamber is known to generate as by-products: lime, sulfur dioxide, various combustion gases, ash, and heat. In order to maximize the efficiency and cost effectiveness of such combustion, it is advantageous to recoup from these combustion by-products as much of the usable material as practical. Of primary importance is the recoupment of heat and lime dust from the flue gas and excess lime from the fluidized bed combustion chamber.
It has been suggested that an additional fluidized bed may serve the dual purpose of collecting excess lime from an adjacent fluidized bed combustion chamber as well as acting as a heat exchange device capturing heat from recirculated flue gas. This type of auxiliary bed is disclosed in U.S. Pat. No. 4,303,023 to Perkins et al. and also shown in U.S. Pat. No. 4,343,246 to Jukkola et al. While these devices do serve the purpose of capturing heat and lime dust they suffer two principal limitations.
The first limitation is the difficulty in operating these types of combustors at reduced levels. Operating at a reduced level is important where it is desirable to reduce the overall reaction rate or where it is desirable to operate the combustor as a calciner only, thereby reducing the contamination of the lime product. The second limitation is the inability of these prior art devices to be adapted to existing conventional pulverized coal boilers.
It can therefore be appreciated by one skilled in the art that a fluidized bed coal combustion device, which has significant calcining capability and which is adaptable to existing conventional coal boilers, would be useful, desirable, and profitable.
It is an object of this invention to provide a coal combustion device which is capable of operating as a calciner during reduced levels of operation. It is a further object of this invention to provide a device which is capable of producing high purity soft burned lime. It is yet a further object of the invention to provide a means for retrofitting conventional coal boilers to obtain the teachings and advantages of the present invention. It is another object of the invention to improve the means by which excess lime generated by a fluidized bed coal combustor is collected and cooled for eventual sale or reuse. It is yet another object of the invention to provide a method whereby a plurality of conventional boilers may be supplied with high quality lime generated by an apparatus incorporating the teachings of the present invention to reduce sulfur dioxide emissions, i.e. provide the conventional boiler with it's own source for lime injection while recouping the heat.